Since at least the early 50's, the demand has been incessant for economical materials capable of performing satisfactorily under increasingly severe operating conditions, notably temperature. For example, and by way of illustration, in the ceramic tile industry frit-firing temperatures have been on the increase in an effort to accomodate new frits and higher furnace loads, this to remain competitive in the market-place. Initially, various manufacturers of furnace rollers for this application used an alloy containing roughly 0.04% C, 0.25% Si, 0.25% Mn, 22.75% Cr, 0.4% Ti, 0.01% Nb, 1.35% Al, 59.5% Ni, 0.35% Co, 0.03% N, 0.001% 0.sub.2, balance essentially iron, the alloy being produced from ingots melted in an air induction furnace. The service life of the rollers lasted up to roughly 18 months at 2060.degree. F. (1127.degree. C.), ultimately failing from oxidation-enhanced stress-rupture failure with fracture being intergranular.
More recently, the rollers have been produced from electric-arc furnace melted, argon-oxygen decarburized (AOD) refined ingots. The composition used differed somewhat from the above, a typical composition being approximately 0.03%C., 0.3% Si, 0.3% Mn, 22.5% Cr, 0.4% Ti, 0.02% Nb, 1.27% Al, 60.8% Ni, 0.08% Co, 0.29% Mo. 0.015% N, less than 0.001% 0.sub.2, and balance essentially iron. At 2050.degree. F. (1121.degree. C.) rollers lasted some 12 months and at times longer. However, at 2130.degree. F. (1165.degree. C.) such rollers manifested failure in 2 months or less.
From our investigation of the problem it would appear that failure is caused by a rather dramatic change in microstructure as temperature is increased. This was not initially or readily apparent since our first approach was to increase the levels of aluminum and chromium to enhance oxidation behavior. But this was not a panacea. In any case, extensive experimentation reflects that circa 2150.degree. C. (1177.degree. C.), and above there is a lack of microstructural control of grain size. It would appear that the M.sub.23 C.sub.6 carbide, stabilized by silicon and molybdenum, but consisting mainly of chromium, begins to redissolve into the matrix. This frees the grain boundaries to migrate under applied stress and results in coarse or massive grains, e.g., one to three grains across the wall thickness, 0.080 in. (2.0 mm), of the rollers. This can be viewed, at least in part, as failure induced by the alternating tensile and compressive stresses set up in the rollers as a consequence of temperature and time. Actually, many grain boundaries appear to be perpendicular to the roller surface and serve as sites for preferential grain boundary oxidation attack which, in turn, leads to premature grain boundary rupture.